Data storage devices of the type known as "Winchester" disc drives, or hard disc drives, are typically utilized as primary data storage devices in modern computer systems. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless direct current (dc) spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 revolutions per minute.
Data are stored on and retrieved from the tracks using an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically comprise an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted.
Thus, when current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
The movement of the heads across the disc surfaces in a disc drive utilizing a voice coil actuator systems is typically under the control of a closed loop servo system. In a closed loop servo system, specific data patterns used to define the location of the heads relative to the disc surface are prerecorded on the discs during the disc drive manufacturing process. These servo data patterns can be recorded exclusively on one surface of one disc and continuously read, or can be recorded at the beginning of each user data recording location and read intermittently between intervals of recording or recovering of user data. Such servo systems are referred to as "dedicated" and "embedded" servo systems, respectively.
It is also common practice in the industry to divide each of the tracks on the disc surface into a number of sectors (also referred to as "data blocks" and "data fields") for the storage of user data. The identity of each sector, and thus the radial and circumferential location of the disc relative to the heads, is determined by prerecorded sector ID information included in the servo data pattern. In typical servo systems, a servo header is recorded at the beginning of each user data sector, which includes, among other information, the track number and sector number, thus providing to the servo system a continually updated status on the location of the actuator relative to the disc.
In some disc drives of the current generation, the data blocks are not only used to store user data, but also can be used to provide tuning information to adapt a read/write channel to the particular combination of recording medium and head for each disc surface. It is common in such disc drives for the data block on the disc surface to include control fields used to automatically adjust the gain of the write and read amplifiers used to control the recording and recovery of user data. Thus, prior to any attempt to access user data, the read/write channel is optimized for each access.
Moreover, data blocks typically include a synchronization (sync) field which enables the disc drive to correctly detect the beginning of the user data stored in the data block. That is, the sync field enables the read/write channel to synchronize with the user data so that the data can be properly retrieved from the data block.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and media have been achieved.
For example, the first rigid disc drives used in personal computers had a data capacity of only 5 megabytes using discs of 133.4 millimeters in diameter (commonly referred to in the industry as the full height, 51/4 inch format). By contrast, personal computers now typically utilize disc drives with data capacities of up to several gigabytes and discs of 95.0 millimeters in diameter (3.74 inch for 31/2 inch format). Portable notebook computers typically use disc drives having about the same recording capacity, but with discs of only 63.5 millimeters in diameter (21/2 inch format). Even smaller disc drives having discs of 45.7 millimeters in diameter (1.8 inch format) have been introduced in Type III Personal Computer Memory Card International Association (PCMCIA) cards, which are credit card sized (85.6 by 54 millimeter) cards popular in notebook computers and other portable electronic devices. Clearly, consumer demands will continue to drive ever greater recording densities for disc drive applications in the foreseeable future.
Likewise, the recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricated using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. As will be recognized, MR heads detect the presence of magnetic flux reversals on the discs by reacting to the presence of flux changes with a proportional change in electrical resistance. The MR read element in such heads is biased with a constant low level dc current, and induced resistance changes are readily detected as sensed changes in the amount of voltage across the MR element.
With increases in data storage density and reductions in flying height of the heads, disc drives are becoming increasingly sensitive to the effects of anomalous conditions caused by defects associated with the media. Particularly, localized anomalies in the media on a disc surface may provide insufficient magnetization characteristics to allow data to be reliably stored and retrieved during the operational life of the disc drive.
Moreover, certain very small defects on the surface of the recording discs can still be large enough to physically contact the MR element of the heads as the discs rotate under the heads. Such contact, while of very short time duration, results in frictional heating of the MR element and the change of temperature brought about by the contact also produces a change in resistance in the MR element. Such events are known as thermal asperities, or TAs, and can significantly distort the readback signal generated by the head.
Similarly, small "hills" and "valleys" in the disc surfaces can also induce thermal asperity events even without physical contact between the MR element and the disc surface. Because the bias current applied to the MR element results in heating of the MR element, a thermal equilibrium is established in which the generated heat in the MR element is constantly dissipated from the MR element through other elements of the head assembly and, to a lesser extent, across the air bearing supporting the slider to the disc itself. Thus, disc surface variations that change the spacing between the MR element and the disc can induce attendant changes in the heat dissipation characteristics of the head, resulting in distortion in the readback signal obtained from the head.
TAs found in disc drives using currently available media are of a size which can span a significant number of bytes; for example, in a disc drive having a data transfer rate of 200 megabits per second (Mbits/sec), uncompensated thermal asperities can typically last from 2 to 5 microseconds, distorting from about 50 to 125 bytes of data. Further, it will be recognized that TAs can grow over time due to factors such as contamination and corrosion of the disc surfaces, which can significantly degrade the capabilities of a disc drive to reliably store and retrieve user data over the operational life of the drive.
Anomalous conditions that cause a distortion of the readback signal corresponding to user data can often be compensated for by the error detection and correction circuitry of the read/write channel; however, anomalous conditions that coincide with control fields used to tune the channel can affect the ability of the channel to be properly set up for receipt of the associated user data. More significantly, an anomalous condition coincident with the sync field can prevent synchronization of the channel with the user data, preventing the disc drive from retrieving the user data stored in a data block altogether.
Defect screening operations are typically performed on disc drives during manufacturing to minimize the effects of anomalous conditions upon disc drive performance. However, the fact that many anomalous conditions associated with disc drives are not initially present in the drives, but rather arise during the operational lives of the drives, undesirably results in the degradation of disc drive performance once the disc drives are placed in user environments. This situation will generally continue as ever greater disc drive data storage and transfer capabilities are achieved.
Accordingly, there is a continual need to minimize the effects of anomalous conditions, such as media defects and thermal asperities, that arise during the operational life of a disc drive that tend to degrade the read performance of the disc drive.